Optimizing an integrated circuit (IC) design comprising at least one wide-gate or wide-bus

ABSTRACT

Systems and techniques are described for optimizing an integrated circuit (IC) design. Some embodiment can perform enumeration on a hardware description language (HDL) description of an IC design to obtain a enumerated IC design that includes at least one technology-independent wide-gate or technology-independent wide-bus, wherein the technology-independent wide-gate represents a logical function that is performed on a variable number of inputs, and wherein the technology-independent wide-bus represents a variable number of signals that are part of a bus. The embodiments can then perform technology-independent IC optimization, synthesis, and technology-dependent IC optimization to obtain an optimized and synthesized IC design.

BACKGROUND Technical Field

This disclosure relates to integrated circuits (ICs). More specifically,this disclosure relates to optimizing an IC design comprising at leastone wide-gate or wide-bus.

Related Art

Advances in process technology and an almost unlimited appetite forconsumer electronics have fueled a rapid increase in the size andcomplexity of IC designs. Software tools can be used to perform variousoperations on IC designs, e.g., creating and editing schematics andlayouts; synthesizing, optimizing, and verifying IC designs; etc. Whatare needed are high-performance user-friendly software tools that enablean IC designer to create IC designs that reduce the overall IC designtime and/or improve the overall quality of results (QoR).

SUMMARY

Some embodiments described herein provide techniques and systems foroptimizing an IC design comprising at least one wide-gate or wide-bus.During operation, an IC design system can perform enumeration on ahardware description language (HDL) description of an IC design toobtain a enumerated IC design that includes at least onetechnology-independent wide-gate or technology-independent wide-bus,wherein the technology-independent wide-gate represents a logicalfunction that is performed on a variable number of inputs, and whereinthe technology-independent wide-bus represents a variable number ofsignals that are part of a bus. Each technology-independent wide-gatecan be a representation of a wide-gate that does not specify all of thecharacteristics that a cell in a cell library specifies, or can be arepresentation of a wide-gate that uses nominal values for one or morecharacteristics of the wide-gate. Each technology-independent wide-buscan be a representation of a wide-bus that does not specify all of thecharacteristics that a bus in a cell library specifies, or can be arepresentation that uses nominal values for one or more characteristicsof the wide-bus.

Next, the IC design system can perform technology-independent ICoptimization on the enumerated IC design to obtain an optimizedenumerated IC design. The IC design system can then determine asynthesized IC design by replacing each technology-independent gate inthe optimized enumerated IC design with a cell or by a network of cells,wherein each cell is selected from a cell library that contains cellsfor a specific semiconductor manufacturing technology. Specifically, thesynthesized IC design can explicitly store (1) information thatbidirectionally links a wide-gate data structure that represents thelogical operation with a set of data structures that represent cellsthat implement the logical operation, and/or (2) information thatbidirectionally links a wide-bus data structure that represents themulti-bit variable with a set of data structures that implement wirescorresponding to the multi-bit variable. Next, the IC design system canperform technology-dependent IC optimization on the synthesized ICdesign to obtain an optimized synthesized IC design.

The bidirectional links can be used to quickly and efficiently access(1) the individual logic gates (or cells) that implement thefunctionality of a given wide-gate, (2) the wide-gate whosefunctionality is being implemented by a given individual logical gate(or cell), (3) the individual wires that implement the functionality ofa given wide-bus, and/or (4) the wide-bus whose functionality is beingimplemented by a given individual wire. Some embodiments describedherein use this ability to quickly and efficiently access informationrelated to wide-gates and/or wide-buses to improve the runtime of theoptimization and/or the quality of results produced by the optimization.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A illustrates how a wide-gate is represented in existingapproaches before and after enumeration.

FIG. 1B illustrates how a wide-bus is represented in existing approachesbefore and after enumeration.

FIG. 1C illustrates a wide-gate construct and illustrates howenumeration can be performed in accordance with some embodimentsdescribed herein.

FIG. 1D illustrates a wide-bus construct and illustrates how enumerationcan be performed in accordance with some embodiments described herein.

FIGS. 1E-1F illustrate how a wide-gate construct can be stored in memoryin accordance with some embodiments described herein.

FIGS. 1G-1H illustrate how a wide-bus construct can be stored in memoryin accordance with some embodiments described herein.

FIGS. 1I-1J illustrate enumerated and synthesized IC design data inexisting approaches.

FIGS. 1K-1L illustrate enumerated and synthesized IC design data inaccordance with some embodiments described herein.

FIG. 2A illustrates a process for using a wide-gate data structure in anIC design flow in accordance with some embodiments described herein.

FIG. 2B illustrates a process for using a wide-bus data structure in anIC design flow in accordance with some embodiments described herein.

FIG. 2C illustrates a process for optimizing an IC design by using awide-gate data structure in accordance with some embodiments describedherein.

FIG. 3A illustrates a process for optimizing an IC design by using awide-bus data structure in accordance with some embodiments describedherein.

FIGS. 3B-3D illustrate an example of multi-bit-mapping aware clockgating in accordance with some embodiments described herein.

FIG. 4A illustrates a slice in accordance with some embodimentsdescribed herein.

FIG. 4B illustrates a process for using slices in an IC design flow inaccordance with some embodiments described herein.

FIG. 4C illustrates an example of how bidirectional links between theslice and each slice instance of the slice can be maintained inaccordance with some embodiments described herein.

FIG. 4D illustrates a process for using slices to perform one or more ICdesign flow operations in accordance with some embodiments describedherein.

FIG. 4E illustrates a process for using slices during IC designoptimization in accordance with some embodiments described herein.

FIG. 5 illustrates an IC design system in accordance with someembodiments described herein.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Overview of IC Design and Manufacturing

IC design software tools can be used to create an IC design. Once the ICdesign is finalized, it can undergo fabrication, packaging, and assemblyto produce IC chips. The overall IC design and manufacturing process caninvolve multiple entities, e.g., one company may create the software fordesigning ICs, another company may use the software to create the ICdesign, and yet another company may manufacture IC chips based on the ICdesign. An IC design flow can include multiple steps, and each step caninvolve using one or more IC design software tools. An improvement toone or more of these steps in the IC design flow results in animprovement to the overall IC design and manufacturing process.Specifically, the improved IC design and manufacturing process canproduce IC chips with a shorter time-to-market (TTM) and/or higherquality of results (QoR). Some examples of IC design steps and theassociated software tools are described below. These examples are forillustrative purposes only and are not intended to limit the embodimentsto the forms disclosed.

Some IC design software tools enable IC designers to describe thefunctionality that the IC designers want to implement. These tools alsoenable IC designers to perform what-if planning to refine functionality,check costs, etc. During logic design and functional verification, theHDL (hardware description language), e.g., SystemVerilog, code can bewritten and the design can be checked for functional accuracy, e.g., thedesign can be checked to ensure that it produces the correct outputs.

During synthesis and design for test, the HDL code can be translated toa netlist using one or more IC design software tools. Further, thenetlist can be optimized for the target technology, and tests can bedesigned and implemented to check the finished chips. During netlistverification, the netlist can be checked for compliance with timingconstraints and for correspondence with the HDL code. Embodimentsdescribed herein can be used in multiple steps of the circuit designflow. Specifically, some embodiments can be used during synthesis,design, and physical implementation.

During design planning, an overall floorplan for the chip can beconstructed and analyzed for timing and top-level routing. Duringphysical implementation, circuit elements can be positioned in thelayout and can be electrically coupled.

During analysis and extraction, the IC design's functionality can beverified at a transistor level and parasitics can be extracted. Duringphysical verification, the design can be checked to ensure correctnessfor manufacturing, electrical issues, lithographic issues, andcircuitry.

During resolution enhancement, geometric manipulations can be performedon the layout to improve manufacturability of the design. During maskdata preparation, the design can be “taped-out” to produce masks whichare used during fabrication.

Wide-Gates and Wide-Buses

The functionality of an IC can be specified by using HDL code. The term“enumeration” refers to the process of constructing a network of gatesthat implements the functionality specified in the HDL code. At a laterpoint in the IC design flow, each gate in the enumerated IC design isimplemented using a cell or a network of cells which performs thelogical function of the gate. A gate is an abstract model of a circuitin a manufactured IC that performs a logical operation or a logicalfunction (e.g., “AND,” “OR,” “XOR,” “4-to-1 multiplexer,” etc.). Thecharacteristics of the actual physical circuit that is being modeled bythe gate, e.g., the power consumption (e.g., leakage power and dynamicpower), the speed (e.g., delay and transition behavior), the size (e.g.,area), etc., are determined by the cell that is used for implementingthe gate.

An IC design can be manufactured using different semiconductormanufacturing technologies, and each semiconductor manufacturingtechnology can correspond to one or more cell libraries that can be usedby an IC design software tool. Each cell in a cell library containsinformation related to a circuit that can be manufactured using thecorresponding semiconductor manufacturing technology. For example, thecell can include the layout information which specifies the physicallocation and the connectivity among different parts (e.g., gate, drain,source, etc.) of a circuit. The cell can also include information aboutthe electrical characteristics, such as the power consumption, speed,and size (i.e., area) of the circuit.

In this disclosure, (1) the term “wide-gate” refers to a gate thatrepresents a logical operation that has more than two, and potentially avariable number of inputs, and (2) the term “wide-bus” refers to acollection of wires that carries information in the IC design. The setof electrical signals carried by a wide-bus corresponds to aninformation unit, and each electrical signal carried by each wire in thewide-bus corresponds to a portion of the information unit. For example,a wide-bus that has 256 wires can carry a 256-bit data value, whereineach wire in the wide-bus carries one bit of the 256-bit value.

In existing approaches, logical operations that have a large number ofinputs are represented by a network of gates in the enumerated ICdesign. This is because the data format or the language that is used forrepresenting the enumerated IC design does not have a construct torepresent a wide-gate that is distinct from the construct that is usedto represent an individual 2-input gate, e.g., a 2-input “AND,” “OR,”“XOR,” gate, etc. FIG. 1A illustrates how a wide-gate is represented inexisting approaches before and after enumeration. HDL code 102 caninclude a statement that describes a logical operation on multipleinputs. For example, HDL code 102 can include a statement that describesan “AND” operation on 8 inputs. After enumeration, the EDA tool cancreate a network of individual gates in the enumerated IC design thatimplements the 8-input “AND” operation. The network of individual gatesis labeled in FIG. 1A as “sea of gates 104” to highlight the fact thatthe enumerated IC design does not explicitly retain information of thefact that all of the gates in the sea of gates 104 correspond to asingle 8-input “AND” operation in the HDL code.

Likewise, in existing approaches, a wide-bus is represented by aplurality of independent wires in the enumerated IC design because thedata format or the language that is used for representing the enumeratedIC design does not have a construct to represent a wide-bus.Specifically, in existing approaches, the data format or the languagethat is used for representing the enumerated IC design only has aconstruct for representing a wire, and this construct is used forrepresenting each individual wire in a wide-bus. FIG. 1B illustrates howa wide-bus is represented in existing approaches before and afterenumeration. HDL code 112 can include a statement that defines amulti-bit variable. For example, HDL code 112 can include the statement“bus[0:47],” which defines a 48-bit variable “bus” that stores a 48-bitvalue. After enumeration, the EDA tool can create plurality of wires 114that represents the 48 bits of the variable “bus” (the plurality ofwires 114 contains 48 wires, wherein each wire represents a distinct bitin the 48-bit variable “bus”). Note that the plurality of wires 114 doesnot explicitly retain information of the fact that all of the wires inthe plurality of wires 114 correspond to a single 48-bit variable in theHDL code.

Some embodiments described herein are partly motivated by the followinginsights. If, throughout the IC design flow, the representation of theIC design explicitly retains information (e.g., by way of a newconstruct) of the fact that certain gates and/or cells in the IC designcorrespond to a single multi-input logical operation (e.g., an 8-input“AND” operation in the HDL code), then this information can be used tosignificantly improve the performance and/or QoR of the IC design.Likewise, if, throughout the IC design flow, the representation of theIC design explicitly retains information (e.g., by way of a newconstruct) of the fact that certain wires in the IC design correspond toa single wide-bus (e.g., a single 48-bit value in the HDL code), thenthis information can be used to significantly improve the performanceand/or QoR of the IC design.

An IC design can be represented using different data formats orlanguages as the IC design progresses through an IC design flow, whereinthe different data formats or languages represent the IC design atdifferent levels of abstraction. In general, higher levels ofabstraction contain fewer details of the IC design when compared withlower levels of abstraction. Typically, the IC design is described at ahigh level of abstraction in the early stages of the IC design flow, andthe level of abstraction becomes progressively lower as the IC designmoves through the IC design flow. For example, toward the beginning ofthe IC design flow, an IC design can be described at a high level ofabstraction by using an HDL which describes the functionality of the ICdesign but does not provide any information about the actual geometricshapes that will be printed on the wafer. Toward the end of the ICdesign flow, the same IC design can be represented in a low level ofabstraction by using a data format or language such as GDSII or OASIS,which provides detailed information of the actual geometric shapes thatare to be printed on the wafer. In between the two ends of the IC designflow, the IC design may be represented in numerous data formats orlanguages that describe the same IC design at different levels ofabstraction.

Some embodiments described herein introduce new constructs to representwide-gates and wide-buses as the IC design moves through multiple stagesof the IC design flow in which the IC design representation progressesfrom a high-level representation to a low-level representation. Notethat it is important to retain the wide-gate and wide-bus informationthrough multiple stages in the IC design flow in order to fully reap theperformance and/or QoR improvements that this information can provide.

As explained above, a wide-gate construct is a distinct construct thatis used to represent a wide-gate in the IC design as the IC design movesthrough multiple stages of the IC design flow. FIG. 1C illustrates awide-gate construct and illustrates how enumeration can be performed inaccordance with some embodiments described herein. HDL code 122 caninclude a statement that describes a logical operation on multipleinputs, e.g., HDL code 122 can include a statement that describes an“AND” operation on 8 inputs. After enumeration, the EDA tool can createwide-gate 124 in the enumerated IC design that implements the 8-input“AND” operation. The wide-gate 124 in the enumerated IC design isrepresented using a new wide-gate construct that is natively supportedby the enumerated IC design data format or the enumerated IC designrepresentation language. Note that the wide-gate construct is distinctfrom a “regular” gate construct that is used for representing 2-inputgates, such as 2-input “AND,” “OR,” “XOR,” gates, etc.

A wide-bus construct is a distinct construct that is used to represent awide-bus in the IC design as the IC design moves through multiple stagesof the IC design flow. FIG. 1D illustrates a wide-bus construct andillustrates how enumeration can be performed in accordance with someembodiments described herein. HDL code 132 can include a statement thatdefines a multi-bit variable, e.g., HDL code 132 can include a statementthat describes the 48-bit variable “bus” as shown in FIG. 1D. Afterenumeration, the EDA tool can create wide-bus 134 in the enumerated ICdesign that implements the wide-bus. The wide-bus 134 in the enumeratedIC design is represented using a new wide-bus construct that is nativelysupported by the enumerated IC design data format or the enumerated ICdesign representation language. Note that the wide-bus construct isdistinct from the construct that is used for representing wires.

FIGS. 1E-1F illustrate how a wide-gate construct can be stored in memoryin accordance with some embodiments described herein. As shown in FIG.1E, a wide-gate data structure 142 can be created for each instance of awide-gate construct in an IC design. Wide-gate data structure 142 cancontain wide-gate data 144, which can include information about thefunctionality of the wide-gate (e.g., a field in wide-gate data 144 canspecify the logical operation that the wide-gate performs), and a uniqueidentifier that enables an EDA tool to uniquely reference this instanceof the wide-gate. At a later point in the IC design flow, this instanceof the wide-gate may be implemented using a network of individual logicgates (e.g., 2-input “AND” gates, 2-input “OR” gates, 2-input “XOR”gates, etc.) that provide the functionality of the wide-gate. FIG. 1Fillustrates an example of how the wide-gate construct can be stored inmemory after the wide-gate has been implemented by a network of logicgates. Specifically, wide-gate data structure 152 can include wide-gatedata 154 information about the functionality of the wide-gate, and aunique identifier that enables an EDA tool to uniquely reference thisinstance of the wide-gate. Wide-gate data structure 152 can also includea set of references (e.g., the references to individual logic gate datastructures 156-1 through 156-n shown in wide-gate data structure 152) tothe individual logic gates in the network of individual logic gates thatimplements the functionality of the wide-gate. Each individual logicgate in the network of individual logic gates that implements thefunctionality of the wide-gate can correspond to a distinct datastructure that is stored in memory. For example, FIG. 1F illustrates anexample of individual logic gate data structure 156-1 that includes (1)individual logic gate data 158, which can include information about thefunctionality of the logic gate, and a unique identifier that enables anEDA tool to uniquely reference this instance of the logic gate, and (2)a reference to the wide-gate data structure 152.

The set of forward references to the individual logic gates (or cells)in the network of individual logic gates (or cells) that implements thefunctionality of the wide-gate (e.g., the references to individual logicgate (or cell) data structures 156-1 through 156-n shown in wide-gatedata structure 152), and the reverse references to the wide-gate (e.g.,the reference to the wide-gate data structure 152 shown in individuallogic gate (or cell) data structure 156-1) provide a bidirectional linkbetween the wide-gate and the individual gates (or cells) thatimplements the functionality of the wide-gate. This bidirectional linkcan be used to quickly and efficiently access (1) the individual logicgates (or cells) that implement the functionality of a given wide-gate,and conversely (2) the wide-gate whose functionality is beingimplemented by a given individual logical gate (or cell). Specifically,a query function in an EDA tool can use the data structures shown inFIG. 1F to (1) look up and return a set of individual logic gate (orcell) identifiers or a set of individual logic gate (or cell) referencesbased on a query that specifies an identifier or a reference for awide-gate, and/or (2) look up and return an identifier or a referencefor a wide-gate based on a query that specifies an identifier or areference for an individual logic gate (or cell).

FIGS. 1G-1H illustrate how a wide-bus construct can be stored in memoryin accordance with some embodiments described herein. As shown in FIG.1G, a wide-bus data structure 162 can be created for each instance of awide-bus construct in an IC design. Wide-bus data structure 162 cancontain wide-bus data 164, which can include a unique identifier thatenables an EDA tool to uniquely reference this instance of the wide-bus.At a later point in the IC design flow, this instance of the wide-busmay be implemented by a set of wires. FIG. 1H illustrates an example ofhow the wide-bus construct can be stored in memory after the wide-bushas been implemented by a set of wires. Specifically, wide-bus datastructure 172 can include wide-bus data 174, which can include a uniqueidentifier that enables an EDA tool to uniquely reference this instanceof the wide-bus. Wide-bus data structure 172 can also include a set ofreferences (e.g., the references to individual wire data structures176-1 through 176-n shown in wide-bus data structure 172) to theindividual wires that are used to implement the functionality of thewide-bus. Each individual wire can correspond to a distinct datastructure that is stored in memory. For example, FIG. 1H illustrates anexample of individual wire data structure 176-1 that includes (1)individual wire data 178, which can include information about the wire(e.g., resistance, capacitance, width, length, metal layer, coordinatesof the end-points of the wires, etc.), and a unique identifier thatenables an EDA tool to uniquely reference this instance of the wire, and(2) a reference to the wide-bus data structure 172.

The set of forward references to the individual wires that implement thewide-bus (e.g., the references to individual wires 176-1 through 176-nshown in wide-bus data structure 172), and the reverse references to thewide-bus (e.g., the reference to the wide-bus data structure 172 shownin individual wire data structure 176-1) provide a bidirectional linkbetween the wide-bus and the individual wires that implements thewide-bus. This bidirectional link can be used to quickly and efficientlyaccess (1) the individual wires that implement the functionality of agiven wide-bus, and conversely (2) the wide-bus whose functionality isbeing implemented by a given individual wire. Specifically, a queryfunction in an EDA tool can use the data structures shown in FIG. 1H to(1) look up and return a set of individual wire identifiers or a set ofindividual wire references based on a query that specifies an identifieror a reference for a wide-bus, and/or (2) look up and return anidentifier or a reference for a wide-bus based on a query that specifiesan identifier or a reference for an individual wire.

FIGS. 1I-1J illustrate enumerated and synthesized IC design data inexisting approaches. In FIGS. 1I-1J, a rectangle represents the entireIC design data, and each circle represents the data for a single gate orcell. Specifically, in FIG. 1I, enumerated IC design data 182-E caninclude data for each gate corresponding to the sea of gates thatimplements an 8-input “AND” operation. During synthesis, each gate inthe enumerated IC design data 182-E can be implemented by one or morecells that are selected from an appropriate cell library to obtainsynthesized IC design data 182-S. As shown in FIG. 1J, synthesized ICdesign data 182-S can include data for each cell that implements the8-input “AND” operation (the data corresponding to each cell is shown asa shaded circle in FIG. 1J). Note that enumerated IC design data 182-Eand synthesized IC design data 182-S do not explicitly store informationof the fact that the gate and cell data shown in FIGS. 1I-1J correspondsto a single 8-input “AND” operation. Likewise, in existing approaches, acorrespondence between a wide-bus and the set of wires that implementthe wide-bus is also not explicitly stored in the enumerated andsynthesized IC design data.

In contrast, FIGS. 1K-1L illustrate enumerated and synthesized IC designdata in accordance with some embodiments described herein. In FIG. 1K, alarge rectangle represents the entire enumerated IC design data 192-E,and the smaller rectangle inside the large rectangle represents awide-gate data structure 194-E that corresponds to a wide-gate, e.g., an8-input “AND” gate. During synthesis, the wide-gate can be implementedby a network of one or more cells that are selected from an appropriatecell library to obtain synthesized IC design data 192-S. As shown inFIG. 1L, synthesized IC design data 192-S can include data for each cellthat implements the wide-gate, and also include wide-gate data structure194-S (the data corresponding to each cell is shown as a shaded circlein FIG. 1L). The bidirectional arrows between each circle thatrepresents a cell data structure and the wide-gate data structure 194-Srepresent bidirectional links that can be implemented by storingappropriate references in the cell and wide-gate data structures. Notethat enumerated IC design data 192-E and synthesized IC design data192-S explicitly store information of the fact that the gate and celldata shown in FIGS. 1K-1L corresponds to a wide-gate, e.g., an 8-input“AND” operation. Likewise, in some embodiments described herein, acorrespondence between a wide-bus and the set of wires that implementthe wide-bus is explicitly stored in the enumerated and synthesized ICdesign data.

FIG. 2A illustrates a process for using a wide-gate data structure in anIC design flow in accordance with some embodiments described herein. Theprocess can begin by receiving an HDL description of an IC design,wherein the HDL description includes an HDL statement that performs alogical operation on more than two inputs (block 202). Next, the processcan enumerate the HDL description to obtain an enumerated IC design,wherein the enumerated IC design includes a wide-gate data structurethat represents the logical operation on more than two inputs (block204). The process can then synthesize the enumerated IC design to obtaina synthesized IC design, wherein the synthesized IC design includes (1)a wide-gate data structure that represents the logical operation on morethan two inputs, (2) a set of individual cell data structures thatimplement the logical operation on more than two inputs, and (3)references that bidirectionally link the wide-gate data structure withthe set of individual cell data structures (block 206).

FIG. 2B illustrates a process for using a wide-bus data structure in anIC design flow in accordance with some embodiments described herein. Theprocess can begin by receiving an HDL description of an IC design,wherein the HDL description includes an HDL statement that defines amulti-bit variable, wherein the multi-bit variable represents a bus(block 212). Next, the process can enumerate the HDL description toobtain an enumerated IC design, wherein the enumerated IC designincludes a wide-bus data structure that represents the multi-bitvariable (block 214). The process can then synthesize the enumerated ICdesign to obtain a synthesized IC design, wherein the synthesized ICdesign includes (1) a wide-bus data structure that represents themulti-bit variable, (2) a set of individual wire data structures thatrepresent individual wires in the bus, and (3) references thatbidirectionally link the wide-bus data structure with the set ofindividual wire data structures (block 216).

IC Design Optimization Using Wide-Gates

A technology-independent wide-gate is a representation of a wide-gatethat does not specify all of the characteristics that a typical cell ina cell library would specify, or that uses nominal values for one ormore characteristics of the wide-gate. For example, atechnology-independent wide-gate may use nominal area and power values,and use a “0” delay value. Likewise, a technology-independent wide-busis a representation of a wide-bus that does not specify all of thecharacteristics that a typical bus in a cell library would specify, orthat uses nominal values for one or more characteristics of thewide-bus. For example, a technology-independent wide-bus may use nominalvalues for the capacitance and resistance of the wide-bus.

FIG. 2C illustrates a process for optimizing an IC design in accordancewith some embodiments described herein. The process can begin byperforming enumeration on an HDL description of an IC design to obtainan enumerated IC design that includes at least onetechnology-independent wide-gate or technology-independent wide-bus,wherein the technology-independent wide-gate represents a logicalfunction that is performed on a variable number of inputs, and whereinthe technology-independent wide-bus represents a variable number ofsignals that are part of a bus (block 222).

Next, the process can perform technology-independent IC optimization onthe enumerated IC design to obtain an optimized enumerated IC design(block 224). The term “technology-independent IC optimization” refers toan optimization that is performed on a network of gates that does nottake into account the characteristics (e.g., power, delay, area) of theactual circuitry that will be manufactured. For example, a given networkof gates may be optimized by replacing the network of gates with asmaller network of gates (i.e., a network of gates that has fewer gates)that has the same logical functionality.

The process can then determine a synthesized IC design by replacing eachtechnology-independent gate in the optimized enumerated IC design with acell or a network of cells, wherein each cell is selected from a celllibrary that contains cells for a specific semiconductor manufacturingtechnology (block 226). Note that a technology-independent gate maycorrespond to a single cell in the cell library, or if the logicalfunctionality of the technology-independent gate is complex, then thetechnology-independent gate may correspond to a network of gates.

Next, the process can perform technology-dependent IC optimization onthe synthesized IC design to obtain an optimized synthesized IC design(operation block 228). The term “technology-dependent IC optimization”refers to an optimization that is performed on a synthesized IC designthat takes into account the characteristics (e.g., power, delay, area)of the actual circuitry that will be manufactured. For example,technology-dependent IC optimization may involve replacing cells withother cells that have the same functionality but that have differentdelay, power, and area characteristics so that the synthesized IC designmeets timing constraints (which ensure that the manufactured IC willoperate correctly at a desired clock speed), power constraints (whichensure that the leakage power consumption of the manufactured IC is lessthan a maximum allowable leakage power consumption and/or the dynamicpower consumption of the manufactured IC is less than a maximumallowable dynamic power consumption), and/or area constraints (whichensure that the total area of the manufactured IC is less than a maximumallowable area).

Multi-Bit-Mapping Aware Clock Gating

Clock gating and mapping of register banks are both optimizations toreduce dynamic power. In clock gating, a clock signal is passed throughcombinational logic before being provided to the clock input ofsequential circuit elements (e.g., registers). The combinational logic“gates” (i.e., blocks) the clock signal when the clock is not required(e.g., when a portion of the IC design has been turned off or disabled),thereby reducing dynamic power. In register bank mapping, multiplesingle-bit registers in the RTL design can be mapped to a singlemulti-bit register library cell. Note that a single multi-bit registeruses the same clock signal for all bits. In some embodiments, clockgating can support multi-bit mapping (i.e., mapping a set of single-bitregisters to one or more multi-bit registers), because clock gatingbefore mapping may make neighboring registers in a bank “uniform,” i.e.,suitable for mapping into a single multi-bit cell, even though the RTLdescription has the neighboring registers as separate one-bit registers.However, the opposite may happen as well, i.e., the most efficient clockgating solution may prevent mapping to multi-bit register cells.

FIG. 3A illustrates a process for optimizing an IC design by using awide-bus data structure in accordance with some embodiments describedherein. The process can begin by selecting a wide-bus (block 302). Next,the process can divide the wide-bus into one or more subsets ofbus-wires, wherein each subset of bus-wires corresponds to a unit ofinformation, e.g., a word (block 304). Note that the process can use thewide-bus data structure (see, e.g., FIG. 1H) to quickly identify thebus-wires that comprise the wide-bus. The process can then optimizeclock gating on each subset of bus-wires (block 306).

Specifically, a wide-bus may correspond to a multi-dimensional unit ofinformation. Therefore, the process may divide the wires of the wide-businto distinct subsets that correspond to each dimension in themulti-dimensional unit of information. For example, suppose an IC designincludes a 64-bit wide-bus that corresponds to four 16-bit values. Inthis example, the process may create four subsets of bus-wires having 16bus-wires each, and optimize clock gating for each of the four subsetsof bus-wires. On the other hand, if the 64-bit wide-bus corresponds to asingle 64-bit value, then the process may create a single set of 64bus-wires, and optimize clock gating for the single set of 64 bus-wires.

Specifically, the process can optimize clock gating for a given subsetof bus-wires as follows. The process can compute local enable functionsfor all registers that correspond to the subset of bus-wires (block306-1). Next, the process can compute global enable functions for allregisters that correspond to the subset of bus-wires (block 306-2). Theprocess can then divide the subset of bus-wires into a set of segmentsbased on a width of a multi-bit register library cell, wherein eachsegment in the set of segments includes a subset of the subset ofbus-wires (block 306-3).

Next, for each segment, the process can determine the strongest commonfactor based on the local enable functions and the global enablefunctions corresponding to the bus-wires in the segment (block 306-4).In some embodiments, the process can determine the strongest commonfactor for a given set of logic functions by simplifying a combinedlogic function that is the logical conjunction (i.e., the logical “AND”operation) of the given set of logic functions. Note that, if all of thelogic functions in the given set of logic functions are identical, thenthe strongest common factor will be one of the logic functions in thegiven set of logic functions.

The process can then add clock gating circuitry in the IC design to gatea clock input of a multi-bit register cell corresponding to the segmentbased on the strongest common factor (block 306-5).

FIGS. 3B-3D illustrate an example of multi-bit-mapping aware clockgating in accordance with some embodiments described herein. FIG. 3Billustrates RTL design 350 (i.e., the IC design before synthesis) thatincludes four registers (R1-R4), wherein each register has a clock inputand an enable input. FIG. 3B also shows the local (E1 and E2) and global(E0) enable signals 352. Registers R1 and R2 in RTL design 350 areenabled by the conjunction of enable signals E0 and E1, and registers R3and R4 in RTL design 350 are enabled by the conjunction of enablesignals E0 and E2. During synthesis, elements in RTL design 350 aremapped to one or more library cells. Some embodiments described hereincan use the wide-bus information stored in the IC design tointelligently map registers in RTL design 350 to specific multi-bitregister cells depending on the capabilities of the multi-bit registercells.

For example, FIG. 3C illustrates IC design after synthesis 360 when thecell library includes multi-bit register cells that do not have distinctenable pins for each bit. As shown in FIG. 3C, registers R1 and R2 canbe mapped to one 2-bit register cell (the top 2-bit register cell), andregisters R3 and R4 can be mapped to another 2-bit register cell (thebottom 2-bit register cell). Clock gating circuitry 362 (which can bedetermined and added to the IC design as explained in reference to FIG.3A) can appropriately gate the clock signal based on local (E1 and E2)and global (E0) enable signals.

Likewise, FIG. 3D illustrates IC design after synthesis 370 when thecell library includes multi-bit register cells that have distinct enablepins for each bit. As shown in FIG. 3D, registers R1-R4 can be mapped toa single 4-bit register cell. Clock gating circuitry 372 (which can bedetermined and added to the IC design as explained in reference to FIG.3A) can appropriately gate the clock signal based on local (E1 and E2)and global (E0) enable signals.

Identification and Preservation of Slices

As mentioned above, a bus is a set of wires in an IC design that carriesunits of information among different parts of the IC design. Each wirein the bus typically carries a portion of a unit of information (e.g., a128-bit word). Since the wires on the bus typically carry differentportions of the same unit of information, they often undergo the samekind of processing. Therefore, each wire in a bus is often connected toa separate instance of the same sequential and/or combinational logic.

Existing synthesis techniques independently optimize each of theseinstances of the same sequential and/or combinational logic, which canincrease the synthesis runtime because each instance is separately andindependently synthesized. Moreover, existing synthesis techniques canresult in suboptimal results in cases where uniform structure andregularity of design implementation is required. A non-uniformimplementation generated by existing techniques can lead to anon-uniform placement, and in some cases this irregularity can lead topoor results.

In this disclosure, the term “slice” can refer to a circuit comprisingsequential and/or combinational logic that occurs at multiple locationswith similar timing context in the IC design. The term “slice instance”in this disclosure refers to an instance of a slice. For example, theterm “slice” can refer to the sequential and/or combinational logic thatis connected to each wire of a bus. Thus, if a bus has 48 wires, andeach wire is connected to a distinct instance of the same sequentialand/or combinational logic, then the sequential and/or combinationallogic can be referred to as a “slice,” and each of the 48 instances ofsequential and/or combinational logic can be referred to as a “sliceinstance.” As another example, one slice can correspond to multiple bitsof the 48-bit bus, and the mapping for one slice can be duplicatedacross the width of the bus, e.g., each slice can be a single 4-bitmulti-bit register, resulting in 12 slice instances across the 48-bitbus. Note that the slices are uniform, and the mapping for one slice isduplicated across the width of the bus. As yet another example, if eachbyte in a 32-byte word is processed in parallel by instances of the samesequential and/or combinational logic, then the sequential and/orcombinational logic can be referred to as a “slice,” and each of the 32instances of sequential and/or combinational logic can be referred to asa “slice instance.” Some embodiments described herein identify slices inan RTL description of an IC design, and use the identified slices toimprove the performance and/or QoR of IC synthesis and/or optimization.

FIG. 4A illustrates a slice in accordance with some embodimentsdescribed herein. IC design 402 can include circuitry 404, bus 406,circuitry 408, and a set of slice instances, such as slice instances 410and 412. Each wire of bus 406 can be electrically connected to aninstance of the same sequential and/or combinational logic, such asslice instance 410 (shown only as a cloud) and slice instance 412 (shownas a cloud with an inset that shows details within the cloud). Note thata slice instance can include only sequential logic, only combinationallogic, or both sequential and combinational logic (e.g., slice instance412).

FIG. 4B illustrates a process for using slices in an IC design flow inaccordance with some embodiments described herein. The process can beginby identifying a slice in an RTL description of an IC design (block422). In some embodiments, each wire of each wide-bus can be assumed tobe connected to a slice instance. In these embodiments, the process canselect a wide-bus, and determine the set of wires that correspond to thewide-bus. Next, for each wire of the wide-bus, the process can identifyand compare the circuitry that is in proximity to the wire. For example,in some embodiments, the process can identify and compare the circuitrythat is within a downstream logic cone of the wire by performing abreadth-first traversal on the net that the wire is connected to in theIC design. If the circuitry in proximity to each wire is the same, thenthe process can conclude that a possible slice has been identified inthe IC design.

Next, the process can analyze the timing context for each potentialslice instance, and group the potential slice instances into sliceinstance groups based on the timing contexts. The term “timing context”refers to timing parameter values (e.g., slack, transition time, etc.)at a set of terminals (e.g., inputs and outputs of logic gates) in aslice instance, or generally in a portion of the IC design. Note thateach slice instance can include multiple terminals, and each terminalcan be associated with a slack value. Specifically, two slice instancescan be assigned to the same slice instance group if the maximumdifference between a slack value of one slice instance and thecorresponding slack value of the other slice instance is less than aslack difference threshold.

Note that the process can create multiple slice instance groups for awide-bus. For example, suppose a wide-bus has 48 wires, and the processhas determined that each wire is connected to a distinct instance of thesame sequential and/or combinational logic. Next, suppose the processcompares the timing contexts of the 48 slice instances and groups thewires into 12 groups of 4 wires each. Then, at this point the processwould have identified 12 distinct slices, with each slice having 4 sliceinstances.

Once the process identifies slices in the IC design, the process canstore this information and use it in subsequent steps (e.g., placement,routing, optimization, legalization, etc.) of the IC design flow forimproving runtime and/or QoR. In some embodiments, any transformation(placing a cell at a particular location, upsizing a cell, downsizing acell, rerouting a net, etc.) that is applied to one of the slices isalso applied to the other slices. Specifically, the process can createand maintain bidirectional links between the slice and each sliceinstance of the slice as the IC design progresses through an IC designflow (block 424). Next, the process can use the bidirectional links topreserve logical and physical similarity among the slice instances ofthe slice as the IC design progresses through the IC design flow, e.g.,by performing the same operation on each slice instance of the slice(block 426).

FIG. 4C illustrates an example of how bidirectional links between theslice and each slice instance of the slice can be maintained inaccordance with some embodiments described herein. Slice data structure432 can include slice data 434 that contains information about theslice, e.g., a unique identifier that enables an EDA tool to uniquelyreference the slice. Slice data structure 432 can also include a set ofreferences to data structures that contain information about the sliceinstances. As shown in FIG. 4C, slice data structure 432 can includereferences to slice instance data structures 436-1 through 436-n. Eachslice instance in the IC design can correspond to a distinct datastructure that is stored in memory. Each slice instance data structure,e.g., slice instance data structure 436-1 can include (1) slice instancedata 438, which can include information about the slice instance (e.g.,a net that describes the circuitry of the slice instance), and a uniqueidentifier that enables an EDA tool to uniquely reference this sliceinstance, and (2) a reference to the slice data structure 432.

The set of forward references to the slice instances (e.g., referencesto slice instance data structures 436-1 through 436-n), and the reversereferences to the slice (e.g., the reference to slice data structure432) provide the bidirectional links between the slice and each sliceinstance of the slice. These bidirectional links can be used to quicklyand efficiently access (1) the slice instances for a given slice, andconversely (2) the slice corresponding to a given slice instance.Specifically, a query function in an EDA tool can use the datastructures shown in FIG. 4C to (1) look up and return a set of sliceinstance identifiers or a set of slice instance references based on aquery that specifies an identifier or a reference for a slice, (2) lookup and return an identifier or a reference for a slice based on a querythat specifies an identifier or a reference for a slice instance, and/or(3) look up and return a set of slice instance identifiers or a set ofslice instance references based on a query that specifies an identifieror a reference for one of the slice instances in the set of sliceinstances.

FIG. 4D illustrates a process for using slices to perform one or more ICdesign flow operations in accordance with some embodiments describedherein. The process can begin by identifying a slice in an RTLdescription of an IC design (block 442). Next, the process can implementa slice instance of the slice to obtain a slice instance implementation(block 444). Implementing a slice instance of the slice can involvereplacing components in the RTL description of the slice withcorresponding components that are at a lower level of abstraction thanRTL (e.g., replacing gates and registers in the RTL description withcells selected from a technology library). The process can thenimplement other slice instances of the slice by replicating the sliceinstance implementation (block 446). Note that replicating the sliceinstance implementation ensures a uniform implementation across allslice instances.

Next, the process can generate a set of placement directives based on arelative ordering of the slice instances of the slice (block 448). Therelative ordering of the slice instances can be derived from a relativeordering of circuitry that is electrically connected to the sliceinstances. For example, the relative ordering of the slice instances canbe derived from the relative ordering of the wires of a wide-bus thatare electrically connected to the slice instances. In particular, ifeach wire of bus 406 in FIG. 4A represents a bit in a word, then thesewires have an inherent relative ordering, e.g., from the leastsignificant bit of the word to the most significant bit of the word, orvice versa. This relative ordering of the bus wires can be used togenerate a relative ordering of the slice instances, e.g., the sliceinstances that are electrically connected to the bus wires can beordered in the same relative order as the bus wires. The process canthen place the slice instance implementations based on the set ofplacement directives to obtain a uniform and regular physicalimplementation of the set of slice instances (block 450).

FIG. 4E illustrates a process for using slices during IC designoptimization in accordance with some embodiments described herein. Theprocess can begin by identifying a slice in an RTL description of an ICdesign (block 452). The process can merge timing constraintscorresponding to a set of slice instances of the slice to obtain mergedtiming constraints (block 454). In some embodiments, the process canmerge a set of timing constraints by selecting the most stringent timingconstraint from the set of timing constraints. For example, suppose aslice instance group includes 20 slice instances, and each sliceinstance has five timing constraints (e.g., each of the five timingconstraints can correspond to a distinct terminal in the sliceinstance). Then, for each of the five timing constraints, the processcan select the corresponding timing constraint that is most stringentacross all of the 20 slice instances. The resulting set of five moststringent timing constraints can be referred to as the set of mergedtiming constraints for slice instance group.

The process can then optimize a single slice instance based on themerged timing constraints by applying one or more optimizations to thesingle slice instance (block 456). Note that the merged timingconstraints correspond to the most stringent timing constraint acrossall slice instances in the slice instance group. Therefore, optimizingthe slice instance to satisfy the merged timing constraints (whichcapture the most stringent timing constraints across all of the sliceinstances) ensures that the optimized slice instance will satisfy timingconstraints across all slice instances.

Next, the process can apply the one or more optimizations to each sliceinstance in the set of slice instances (block 458). For example, if theone or more optimizations involves upsizing or downsizing certain cellsin the slice instance, then the process can upsize/downsizecorresponding cells in each slice instance.

IC Design System

The term “IC design system” generally refers to a hardware-based systemthat facilitates designing ICs. FIG. 5 illustrates an IC design systemin accordance with some embodiments described herein. IC design system502 can include processor 504, memory 506, and storage device 508.Specifically, memory locations in memory 506 can be addressable byprocessor 504, thereby enabling processor 504 to access (e.g., viaload/store instructions) and manipulate (e.g., via logical/floatingpoint/arithmetic instructions) the data stored in memory 506. IC designsystem 502 can be coupled to display device 514, keyboard 510, andpointing device 512. Storage device 508 can store operating system 516,IC design tool 518, and data 520. Data 520 can include input required byIC design tool 518 and/or output generated by IC design tool 518.

IC design system 502 may automatically (or with user help) perform oneor more operations that are implicitly or explicitly described in thisdisclosure. Specifically, IC design system 502 can load IC design tool518 into memory 506, and IC design tool 518 can then be used to create,edit, view, and optimize IC designs.

The above description is presented to enable any person skilled in theart to make and use the embodiments. Various modifications to thedisclosed embodiments will be readily apparent to those skilled in theart, and the general principles defined herein are applicable to otherembodiments and applications without departing from the spirit and scopeof the present disclosure. Thus, the present invention is not limited tothe embodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein.

The data structures and code described in this disclosure can bepartially or fully stored on a computer-readable storage medium and/or ahardware module and/or hardware apparatus. A computer-readable storagemedium includes, but is not limited to, volatile memory, non-volatilememory, magnetic and optical storage devices such as disk drives,magnetic tape, CDs (compact discs), DVDs (digital versatile discs ordigital video discs), or other media, now known or later developed, thatare capable of storing code and/or data. Hardware modules or apparatusesdescribed in this disclosure include, but are not limited to,application-specific integrated circuits (ASICs), field-programmablegate arrays (FPGAs), dedicated or shared processors, and/or otherhardware modules or apparatuses now known or later developed.

The methods and processes described in this disclosure can be partiallyor fully embodied as code and/or data stored in a computer-readablestorage medium or device, so that when a computer system reads andexecutes the code and/or data, the computer system performs theassociated methods and processes. The methods and processes can also bepartially or fully embodied in hardware modules or apparatuses, so thatwhen the hardware modules or apparatuses are activated, they perform theassociated methods and processes. Note that the methods and processescan be embodied using a combination of code, data, and hardware modulesor apparatuses.

The foregoing descriptions of embodiments of the present invention havebeen presented only for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

What is claimed is:
 1. A non-transitory computer-readable storage mediumstoring instructions for an integrated circuit (IC) design tool that,when executed by a computer, cause the computer to perform a method foroptimizing an IC design, the method comprising: performing enumerationon a hardware description language (HDL) description of the IC design toobtain an enumerated IC design that includes at least onetechnology-independent wide-gate or technology-independent wide-bus,wherein the technology-independent wide-gate represents a logicalfunction that is performed on a variable number of inputs, and whereinthe technology-independent wide-bus represents a variable number ofsignals that are part of a bus; performing technology-independent ICoptimization on the enumerated IC design to obtain an optimizedenumerated IC design; determining a synthesized IC design by replacingeach technology-independent gate in the optimized enumerated IC designwith a cell or by a network of cells, wherein each cell is selected froma cell library that contains cells for a specific semiconductormanufacturing technology; wherein when the enumerated IC design includesthe technology-independent wide-gate, the synthesized IC design stores(1) a wide-gate data structure that represents the logical function, (2)a set of individual cell data structures that implement the logicalfunction, and (3) references that bidirectionally link the wide-gatedata structure with the set of individual cell data structures; whereinwhen the enumerated IC design includes the technology-independentwide-bus, the synthesized IC design stores (1) a wide-bus data structurethat corresponds to the variable number of signals, (2) a set ofindividual wire data structures that correspond to individual signals inthe variable number of signals, and (3) references that bidirectionallylink the wide-bus data structure with the set of individual wire datastructures; and performing technology-dependent IC optimization on thesynthesized IC design to obtain an optimized synthesized IC design. 2.The non-transitory computer-readable storage medium of claim 1, whereineach technology-independent wide-gate is a representation of a wide-gatethat does not specify all of the characteristics that a cell in a celllibrary specifies, or is a representation of a wide-gate that usesnominal values for one or more characteristics of the wide-gate.
 3. Thenon-transitory computer-readable storage medium of claim 1, wherein eachtechnology-independent wide-bus is a representation of a wide-bus thatdoes not specify all of the characteristics that a bus in a cell libraryspecifies, or is a representation that uses nominal values for one ormore characteristics of the wide-bus.
 4. A method for optimizing anintegrated circuit (IC) design, the method comprising: performingenumeration on a hardware description language (HDL) description of anIC design to obtain an enumerated IC design that includes at least onetechnology-independent wide-gate or technology-independent wide-bus,wherein the technology-independent wide-gate represents a logicalfunction that is performed on a variable number of inputs, and whereinthe technology-independent wide-bus represents a variable number ofsignals that are part of a bus; performing technology-independent ICoptimization on the enumerated IC design to obtain an optimizedenumerated IC design; determining a synthesized IC design by replacingeach technology-independent gate in the optimized enumerated IC designwith a cell or by a network of cells, wherein each cell is selected froma cell library that contains cells for a specific semiconductormanufacturing technology; wherein when the enumerated IC design includesthe technology-independent wide-gate, the synthesized IC design stores(1) a wide-gate data structure that represents the logical function, (2)a set of individual cell data structures that implement the logicalfunction, and (3) references that bidirectionally link the wide-gatedata structure with the set of individual cell data structures; whereinwhen the enumerated IC design includes the technology-independentwide-bus, the synthesized IC design stores (1) a wide-bus data structurethat corresponds to the variable number of signals, (2) a set ofindividual wire data structures that correspond to individual signals inthe variable number of signals, and (3) references that bidirectionallylink the wide-bus data structure with the set of individual wire datastructures; and performing technology-dependent IC optimization on thesynthesized IC design to obtain an optimized synthesized IC design. 5.The method of claim 4, wherein each technology-independent wide-gate isa representation of a wide-gate that does not specify all of thecharacteristics that a cell in a cell library specifies, or is arepresentation of a wide-gate that uses nominal values for one or morecharacteristics of the wide-gate.
 6. The method of claim 4, wherein eachtechnology-independent wide-bus is a representation of a wide-bus thatdoes not specify all of the characteristics that a bus in a cell libraryspecifies, or is a representation that uses nominal values for one ormore characteristics of the wide-bus.
 7. An integrated circuit (IC)design system, comprising: a processor; and a non-transitorycomputer-readable storage medium storing instructions that, whenexecuted by the processor, cause the IC design system to perform amethod for optimizing an IC design, the method comprising: performingenumeration on a hardware description language (HDL) description of anIC design to obtain an enumerated IC design that includes at least onetechnology-independent wide-gate or technology-independent wide-bus,wherein the technology-independent wide-gate represents a logicalfunction that is performed on a variable number of inputs, and whereinthe technology-independent wide-bus represents a variable number ofsignals that are part of a bus; performing technology-independent ICoptimization on the enumerated IC design to obtain an optimizedenumerated IC design; determining a synthesized IC design by replacingeach technology-independent gate in the optimized enumerated IC designwith a cell or by a network of cells, wherein each cell is selected froma cell library that contains cells for a specific semiconductormanufacturing technology; wherein when the enumerated IC design includesthe technology-independent wide-gate, the synthesized IC design stores(1) a wide-gate data structure that represents the logical function, (2)a set of individual cell data structures that implement the logicalfunction, and (3) references that bidirectionally link the wide-gatedata structure with the set of individual cell data structures; whereinwhen the enumerated IC design includes the technology-independentwide-bus, the synthesized IC design stores (1) a wide-bus data structurethat corresponds to the variable number of signals, (2) a set ofindividual wire data structures that correspond to individual signals inthe variable number of signals, and (3) references that bidirectionallylink the wide-bus data structure with the set of individual wire datastructures; and performing technology-dependent IC optimization on thesynthesized IC design to obtain an optimized synthesized IC design. 8.The IC design system of claim 7, wherein each technology-independentwide-gate is a representation of a wide-gate that does not specify allof the characteristics that a cell in a cell library specifies, or is arepresentation of a wide-gate that uses nominal values for one or morecharacteristics of the wide-gate.
 9. The IC design system of claim 7,wherein each technology-independent wide-bus is a representation of awide-bus that does not specify all of the characteristics that a bus ina cell library specifies, or is a representation that uses nominalvalues for one or more characteristics of the wide-bus.